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It has removed waste down to regulatory standards in seven of them, and Tank C-108 would be the eighth. Tank C-108 also is one of 16 tanks that a court-enforced consent decree requires to be emptied by fall 2014.

Although it's been five years since a tank has been declared empty, significant amounts of waste have been removed from underground tanks. Almost 2 million gallons of radioactive waste have been retrieved since 2002 from single-shell tanks and transferred to newer, double-shell tanks to await treatment.

Among the challenges of emptying the tanks is the different consistencies of waste, which range from the density of peanut butter to gravel to low-strength concrete.

Many of the tanks have required more than one technology to remove the waste.

That was the case for Tank C-108.

A modified sluicing system was used in the tank more than four years ago when it had about 66,000 gallons of solid waste. Pumpable liquids already have been removed from all single-shell tanks.

Modified sluicing, which lowers two high-pressure nozzles into a tank to spray liquid on waste and move it toward a central pump, was able to remove all but 6,800 gallons of waste in a hard layer at the bottom of Tank C-108.

To remove as much of that layer as possible, contractor Washington River Protection Solutions turned to a chemical dissolution system.

Work began in October to soak the remaining waste in water, which helped remove water-soluble salts. In addition, concentrated sodium hydroxide, a common industrial caustic already used in newer tanks to preserve the correct pH level, also was added to the tank.

The caustic converts the aluminum gibbsite salts in the waste to sodium aluminate, which can be removed with water.

Pumping continues on the tank.

"We are just not getting any waste material anymore," said Stacy Charboneau, deputy manager for the DOE Office of River Protection.

DOE estimates that about 500 to 600 cubic feet of waste remains in the tank. The regulatory requirement is that just 360 cubic feet remain, which would be about 1 inch of waste if it were spread evenly over the bottom of the tank.

Instead, photos and videos taken inside the enclosed tank show it clings in places to the walls of the tank and at the elbows at the bottom of the tank.

DOE also is looking at the makeup of the waste to see if it can make a case that there is limited risk in the waste that remains, which is mostly aluminum and has limited radiological content.

The consent decree allows DOE to make the case to the state that continuing to a third technology does not make sense, said Joanne Grindstaff, DOE project director for tank retrieval and closure.

The chemical dissolution took longer than laboratory tests indicated it would, but it appears to have been successful, Grindstaff said. In the future, DOE will plan for longer water presoaks.

Washington River Protection Solutions also is emptying Tank C-112 using an updated modified sluicing system that telescopes to allow one spray nozzle to have greater reach and get much closer to all the waste.

Retrieval started in December with the enhanced reach sluicing system and 63,500 gallons, or about 63 percent of the waste, has been removed, but retrieval has started to slow, Charboneau said.

At the end of the month, Washington River Protection Solutions also expects to return to work in Tank C-107, where a new robotic arm, the Mobile Arm Retrieval System, was being used. The system, called MARS for short, is the largest and most robust technology used to date at the Hanford tank farms.

Work stopped temporarily when a pump used to remove liquid waste from a double-shell tank needed to be replaced. MARS uses liquid waste for sluicing rather than water to prevent new waste from being created.

Plans also call for chemical dissolution to be used in two more tanks this year and for MARS technology that will use a vacuum system rather than a sluicing system to be prepared for installation in Tank C-105. Construction has started to install enhanced reach sluicing systems in Tanks C-101 and C-102.